It is well recognized in the art that the full potency of a given organic herbicide is not generally attained without the inclusion of various adjuvants, and adjuvant being broadly defined as any substance which enhances the effectiveness of the herbicide. Thus, for example, through proper formulation with an activity-increasing adjuvant, the damage inflicted upon a particular plant species by an herbicide can be amplified many fold. Such an activity-increasing adjuvant does not generally have biological activity on its own but only brings out the activity of the herbicide.
An example of the aforementioned activity-increasing adjuvants is the class of surfactants known as silicone glycols. These liquids have been shown to enhance the efficacy of various herbicides. L. L. Jansen (Weed Science, v. 21, pages 130-135, March, 1973) examined the effect of adding various silicone glycol adjuvants to different herbicides and found that these adjuvants were superior to a standard organic surfactant in eight plant species. In this study, cationic amino silicone surfactants were also evaluated, but found to be less effective than the organic material. In any event, no specific structures of the silicone compounds were provided in this paper.
Great Britian Pat. No. 1,255,249 to Dow Corning Corporation again discloses herbicide compositions employing silicone glycol copolymers. Here, general utility of a large number of adjuvants is professed, as exemplified by two generic silicone glycol formulas which embrace structures having both diorganosiloxane units and alkyl-glycol siloxane units. There is also provided a wide-ranging list of suitable herbicides. This reference, however, provides little direction to those skilled in the art as to which particular silicone glycol structures are to be advantageously combined with specific herbicides, save for two examples employing a triazine herbicide in conjunction with an adjuvant having 1.8 siloxy units and bearing a glycol chain having 12 ethylene oxide units.
In addition to the herbicidal enhancement provided by the activity-increasing adjuvants discussed above, it is often important that herbicide formulations retain a significant degree of activity when plants treated therewith are exposed to rain shortly after application, this being a definition of the degree of "rainfastness." This is particularly critical in the case of water-soluble postemergent herbicides, such as acifluorfen-sodium, which are easily washed away by rainfall occurring within about six hours of application. Typically, this problem is currently addressed by inclusion of another class or adjuvants in the herbicide formulation, namely "sticking agents." The main function of these materials, as the appellation implies, is to impart an increased measure of adhesion of the herbicide composition to plant foliage and thus prevent premature washing away should precipitation occur after the plants are sprayed. The sticking agents are usually polymeric compounds which are generally water-insoluble and tacky in nature.
Neither of the above references addresses the issue of rainfastness nor do these references suggest to those of ordinary skill in the art how particular silicone glycol compounds may be employed to provide both enhanced herbicidal activity to particular herbicides as well as increased rainfastness in the very same formulation, without resorting to the use of additional sticking agents.
In a copending application, Ser. No. 07/274,067, filed on Nov. 21, 1988, assigned to the assignee of the present invention and hereby incorporated by reference, it was shown that the rainfastness of a postemergent herbicide can be synergistically increased by the inclusion of a combination of a silicone glycol, and a dispersant for the silicone glycol, in the herbicide composition. Quite surprisingly, the improvement in rainfastness resulted only when the silicone glycol had an average of four or five ethylene oxide units in its glycol chain. Neither this silicone glycol nor the dispersants therefor, provided rainfastness when used as the sole adjuvant in conjunction with the herbicide.